Heretofore when filling a storage tank on a maritime tanker, in an on-shore tank battery, or the like, hydrocarbon vapor may be generated due to the temperature difference between (1) the hydrocarbonaceous liquid which is entering and filling the storage tank and (2) the gas, hereinafter referred to as "atmospheric gas", occupying the empty storage tank before filling of same. In many locations of the world the temperature of the hydrocarbonaceous liquid being loaded into the storage tank greatly exceeds the temperature of the atmospheric gas at the time of filling. Hydrocarbon vapor can be generated because such vapor as emitted from the hydrocarbonaceous liquid near the liquid-gas interface is heated and, therefore, less dense than the overlying atmospheric gas. This causes vapor-gas interchange that lifts the evaporated hydrocarbon vapor higher into the overlying gas space and returns atmospheric gas to the vicinity of the liquid-gas interface. This mechanism tends to reduce the partial pressure of hydrocarbon vapor in the gas near the oil-gas interface and thereby accelerates the rate of hydrocarbon vapor diffusion into the overlying gas. Accordingly, additional evaporation of hydrocarbons is accelerated.
Heretofore, it has been proposed that, when filling a storage tank with the hydrocarbonaceous liquid having a temperature greater than the ambient temperature within the storage tank, the amount of hydrocarbon vapor emission can be reduced by providing in the storage tank to be filled an essentially inert gas which is at a temperature which matches or exceeds the temperature of the hydrocarbonaceous liquid introduced into the storage tank during the filling process. This is fully and completely disclosed in co-pending U.S. Pat. Application, Ser. No. 07/497278, filed Mar. 22, 1990, and having a common inventor and assignee.
It has since been found, in accordance with this invention, that the method for reducing hydrocarbon vapor emission from a storage tank during the filling of same can be even further improved (reduced) when employing an essentially inert gas at a temperature which matches or exceeds the temperature of the hydrocarbonaceous liquid entering the storage tank during the filling process. The improvement is obtained by employing more than one storage tank and removing at least a portion of said temperature controlled inert gas from a first storage tank which is being filled and passing such removed inert gas into at least one second storage tank that is later to be filled with similar liquid.